Transcript
LM4911 Stereo 40mW Low Noise Headphone Amplifier with Selectable Capacitive Coupled or OCL Output General Description
Key Specifications
The LM4911 is an stereo audio power amplifier capable of delivering 40mW per channel of continuous average power into a 16Ω load or 25mW per channel into a 32Ω load at 1% THD+N from a 3V power supply. Boomer audio power amplifiers were designed specifically to provide high quality output power with a minimal amount of external components. Since the LM4911 does not require bootstrap capacitors or snubber networks, it is optimally suited for low-power portable systems. In addition, the LM4911 may be configured for either single-ended capacitively coupled outputs or for OCL outputs (patent pending). The LM4911 features a low-power consumption shutdown mode and a power mute mode that allows for faster turn on time with less than 1mV voltage change at outputs on release. Additionally, the LM4911 features an internal thermal shutdown protection mechanism.
n PSRR at 217 Hz and 1kHz 65dB (typ) n Output Power at 1kHz with VDD = 2.4V, 1% THD+N into a 16Ω load 25mW (typ) n Output Power at 1kHz with VDD = 3V, 1% THD+N into a 16Ω load 40mW (typ) n Shutdown Current 2.0µA (max) n Output Voltage change on release from Shutdown VDD = 2.4V, RL = 16Ω (C-Coupled) 1mV (max) n Mute Current 100µA (max)
The LM4911 is unity gain stable and may be configured with external gain-setting resistors.
Features OCL or capacitively coupled outputs (patent pending) External gain-setting capability Available in space-saving MSOP package Ultra low current shutdown mode Mute mode allows fast turn-on (1ms) with less than 1mV change on outputs n 2V - 5.5V operation n Ultra low noise n n n n n
Applications n Portable CD players n PDAs n Portable electronics devices
Block Diagram
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FIGURE 1. Block Diagram
Boomer ® is a registered trademark of National Semiconductor Corporation.
© 2002 National Semiconductor Corporation
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LM4911 Stereo 40mW Low Noise Headphone Amplifier with Selectable Capacitive Coupled or OCL Output
July 2002
LM4911
Typical Application
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FIGURE 2. Typical Capacitive Coupled Output Configuration Circuit
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FIGURE 3. Typical OCL Output Configuration Circuit
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LM4911
Connection Diagrams MSOP Package
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Top View Order Number LM4911MM See NS Package Number MUB10A MSOP Marking
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Top View G-Boomer Family A3 - LM4911MM
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LM4911
Absolute Maximum Ratings
Thermal Resistance
(Note 2)
If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Supply Voltage
6.0V
Storage Temperature
Temperature Range
-0.3V to VDD + 0.3V
Power Dissipation (Note 3)
TMIN ≤ TA ≤ TMAX
Internally Limited
ESD Susceptibility (Note 4)
2000V
ESD Susceptibility (Note 5)
200V
Junction Temperature
56˚C/W 190˚C/W
Operating Ratings
−65˚C to +150˚C
Input Voltage
θJC (MSOP) θJA (MSOP)
−40˚C ≤ T
A
≤ 85˚C
2V ≤ VCC ≤ 5.5V
Supply Voltage (VDD)
150˚C
Electrical Characteristics VDD = 5V (Notes 1, 2) The following specifications apply for VDD = 5V, RL = 16Ω, and CB = 4.7µF unless otherwise specified. Limits apply to TA = 25˚C. Symbol
Parameter
Conditions
LM4911 Typ (Note 6)
Limit (Note 7)
2
5
Units (Limits)
IDD
Quiescent Power Supply Current
VIN = 0V, IO = 0A
ISD
Shutdown Current
VSHUTDOWN = GND
0.1
mA (max) µA(max)
IM
Mute Current
VMUTE = VDD, C-Coupled
100
µA(max)
VSDIH
Shutdown Voltage Input High
1.8
V
VSDIL
Shutdown Voltage Input Low
0.4
V
VMIH
Mute Voltage Input High
1.8
V
VMIL
Mute Voltage Input Low
0.4
V
PO
Output Power
THD ≤ 1%; f=1 kHZ OCL, RL = 16Ω
80
OCL, RL = 32Ω
80
C-CUPL, RL = 16Ω
145
mW
C-CUPL, RL = 32Ω
85
VON
Output Noise Voltage
BW = 20Hz to 20kHz, A-weighted
10
µV
PSRR
Power Supply Rejection Ratio
VRIPPLE = 200mV sine p-p f = 1kHz (Note 9)
65
dB
Electrical Characteristics VDD = 3.0V (Notes 1, 2) The following specifications apply for VDD = 3.0V, RL = 16Ω, and CB = 4.7µF unless otherwise specified. Limits apply to TA = 25˚C. Symbol
Parameter
Conditions
LM4911 Typ (Note 6)
Limit (Note 7)
Units (Limits)
IDD
Quiescent Power Supply Current
VIN = 0V, IO = 0A
1.5
3
ISD
Shutdown Current
VSHUTDOWN = GND
0.1
2.0
mA (max) µA(max)
IM
Mute Current
VMUTE = VDD, C-Coupled
50
100
µA(max)
PO
Output Power
THD = 1%; f = 1kHz R = 16Ω
40
R = 32Ω
25
mW
VNO
Output Noise Voltage
BW = 20 Hz to 20kHz, A-weighted
10
µV
PSRR
Power Supply Rejection Ratio
VRIPPLE = 200mV sine p-p
65
dB
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The following specifications apply for VDD = 2.4V, RL = 16Ω, and CB = 4.7µF unless otherwise specified. Limits apply to TA = 25˚C. Symbol
Parameter
Conditions
LM4911 Typ (Note 6)
Limit (Note 7)
Units (Limits)
IDD
Quiescent Power Supply Current
VIN = 0V, IO = 0A
1.5
3
mA (max)
ISD
Shutdown Current
VSHUTDOWN = GND
0.1
2.0
µA(max)
IM
Mute Current
VMUTE = VDD, C-Coupled
40
80
µA(max)
THD = 1%; f = 1kHz PO
Output Power
R = 16Ω
VNO
Output Noise Voltage
BW = 20 Hz to 20kHz, A-weighted
10
µV
PSRR
Power Supply Rejection Ratio
VRIPPLE = 200mV sine p-p
65
dB
TWU
Wake Up Time from Shutdown
OCL C-Coupled, CO = 100µF
0.5 2
s
VOSD
Output Voltage Change on Release from Shutdown
C-Coupled, CO = 100µF
TUM
Time to Un-Mute
C-Coupled, CO = 100µF
R = 32Ω
25
mW
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0.01
1
mV (max)
0.02
s (max)
Note 1: All voltages are measured with respect to the GND pin unless otherwise specified. Note 2: : Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is functional but do not guarantee specific performance limits. Electrical Characteristics state DC and AC electrical specifications under particular test conditions which guarantee specific performance limits. This assumes that the device is within the Operating Ratings. Specifications are not guaranteed for parameters where no limit is given, however, the typical value is a good indication of device performance. Note 3: : The maximum power dissipation must be derated at elevated temperatures and is dictated by TJMAX, θJA, and the ambient temperature, TA. The maximum allowable power dissipation is PDMAX = (TJMAX - TA)/ θJA or the number given in Absolute Maximum Ratings, whichever is lower. For the LM4911, see power derating currents for more information. Note 4: Human body model, 100pF discharged through a 1.5kΩ resistor. Note 5: Machine Model, 220pF-240pF discharged through all pins. Note 6: Typicals are measured at 25˚C and represent the parametric norm. Note 7: Limits are guaranteed to National’s AOQL (Average Outgoing Quality Level). Note 8: Datasheet min/max specification limits are guaranteed by design, test, or statistical analysis. Note 9: 10Ω Terminated input.
External Components Description Components
(Figure 2) Functional Description
1.
RI
Inverting input resistance which sets the closed-loop gain in conjunction with Rf. This resistor also forms a high-pass filter with Ci at fc = 1/(2πRiCi).
2.
CI
Input coupling capacitor which blocks the DC voltage at the amplifier’s input terminals. Also creates a high-pass filter with Ri at fc = 1/(2πRiCi). Refer to the section Proper Selection of External Components, for an explanation of how to determine the value of Ci.
3.
Rf
Feedback resistance which sets the closed-loop gain in conjunction with Ri.
4.
CS
Supply bypass capacitor which provides power supply filtering. Refer to the Power Supply Bypassing section for information concerning proper placement and selection of the supply bypass capacitor.
5.
CB
Bypass pin capacitor which provides half-supply filtering. Refer to the section, Proper Selection of Proper Components, for information concerning proper placement and selection of CB
6.
Co
Output coupling capacitor which blocks the DC voltage at the amplifier’s output. Forms a high pass filter with RL at fo = 1/(2πRLCo)
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LM4911
Electrical Characteristics VDD = 2.4V (Notes 1, 2)
LM4911
Typical Performance Characteristics THD+N vs Frequency
THD+N vs Frequency
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THD+N vs Frequency
THD+N vs Frequency
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THD+N vs Frequency
THD+N vs Frequency
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LM4911
Typical Performance Characteristics
(Continued)
THD+N vs Frequency
THD+N vs Frequency
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THD+N vs Frequency
THD+N vs Frequency
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THD+N vs Frequency
THD+N vs Frequency
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LM4911
Typical Performance Characteristics
(Continued)
THD+N vs Output Power
THD+N vs Output Power
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THD+N vs Output Power
THD+N vs Output Power
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THD+N vs Output Power
THD+N vs Output Power
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LM4911
Typical Performance Characteristics
(Continued)
THD+N vs Output Power
THD+N vs Output Power
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Output Resistance vs Load Resistance
Output Power vs Supply Voltage
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Output Power vs Supply Voltage
Output Power vs Supply Voltage
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LM4911
Typical Performance Characteristics
(Continued)
Output Power vs Supply Voltage
Output Power vs Load Resistance
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Output Power vs Load Resistance
Power Dissipation vs. Output Power
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Power Dissipation vs. Output Power
Power Dissipation vs Output Power
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LM4911
Typical Performance Characteristics
(Continued)
Channel Seperation
Channel Seperation
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Channel Seperation
Channel Seperation
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Channel Seperation
Channel Seperation
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LM4911
Typical Performance Characteristics
(Continued)
Power Supply Rejection Ratio
Power Supply Rejection Ratio
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Power Supply Rejection Ratio
Power Supply Rejection Ratio
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Power Supply Rejection Ratio
Power Supply Rejection Ratio
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LM4911
Typical Performance Characteristics
(Continued)
Power Supply Rejection Ratio
Power Supply Rejection Ratio
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Frequency Response vs Input Capacitor Size
Frequency Response vs Input Capacitor Size
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Supply Voltage vs Supply Current
Open Loop Frequency Response
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LM4911
Typical Performance Characteristics
(Continued)
Clipping Voltage vs Supply Voltage
Noise Floor
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Shutdown Hysteresis Voltage, Vdd=5V
Shutdown Hysteresis Voltage, Vdd=3V
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Power Derating Curve
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Power dissipation is a major concern when using any power amplifier and must be thoroughly understood to ensure a successful design. When operating in capacitor-coupled mode, Equation 1 states the maximum power dissipation point for a single-ended amplifier operating at a given supply voltage and driving a specified output load.
AMPLIFIER CONFIGURATION EXPLANATION As shown in Figure 1, the LM4911 has three operational amplifiers internally. Two of the amplifier’s have externally configurable gain while the other amplifier is internally fixed at the bias point acting as a unity-gain buffer. The closedloop gain of the two configurable amplifiers is set by selecting the ratio of Rf to Ri. Consequently, the gain for each channel of the IC is
PDMAX = (VDD)
2
/ (2π2RL)
(1)
Since the LM4911 has two operational amplifiers in one package, the maximum internal power dissipation point is twice that of the number which results from Equation 1. From Equation 1, assuming a 3V power supply and an 32Ω load, the maximum power dissipation point is 14mW per amplifier. Thus the maximum package dissipation point is 28mW.
AVD = -(Rf / Ri) By driving the loads through outputs VoA and VoB with VoC acting as a buffered bias voltage the LM4911 does not require output coupling capacitors. The classical singleended amplifier configuration where one side of the load is connected to ground requires large, expensive output coupling capacitors.
When operating in OCL mode, the maximum power dissipation increases due to the use of the third amplifier as a buffer and is given in Equation 2:
A configuration, such as the one used in the LM4911, has a major advantage over single supply, single-ended amplifiers. Since the outputs VoA, VoB, and VoC are all biased at 1/2 VDD, no net DC voltage exists across each load. This eliminates the need for output coupling capacitors which are required in a single-supply, single-ended amplifier configuration. Without output coupling capacitors in a typical singlesupply, single-ended amplifier, the bias voltage is placed across the load resulting in both increased internal IC power dissipation and possible loudspeaker damage.
PDMAX = 4(VDD)
2
/ (π2RL)
(2)
The maximum power dissipation point obtained from either Equation 1 or 2 must not be greater than the power dissipation that results from Equation 3: PDMAX = (TJMAX - TA) / θJA
OUTPUT CAPACITOR vs. CAPACITOR COUPLED The LM4911 is a stereo audio power amplifier capable of operating in two distinct output modes: capacitor coupled (C-CUPL) or output capacitor-less (OCL). The LM4911 may be run in capacitor coupled mode by using a coupling capacitor on each single-ended output (VoA and VoB) and connecting VoC to ground. This output coupling capacitor blocks the half supply voltage to which the output amplifiers are typically biased and couples the audio signal to the headphones or other single-ended (SE) load. The signal return to circuit ground is through the headphone jack’s sleeve. The LM4911 can also eliminate these output coupling capacitors by running in OCL mode. Unless shorted to ground, VoC is internally configured to apply a 1⁄2 Vdd bias voltage to a stereo headphone jack’s sleeve. This voltage matches the bias voltage present on VoA and VoB outputs that drive the headphones. The headphones operate in a manner similar to a bridge-tied load (BTL). Because the same DC voltage is applied to both headphone speaker terminals this results in no net DC current flow through the speaker. AC current flows through a headphone speaker as an audio signal’s output amplitude increases on the speaker’s terminal. The headphone jack’s sleeve is not connected to circuit ground when used in OCL mode. Using the headphone output jack as a line-level output will place the LM4911’s 1⁄2 Vdd bias voltage on a plug’s sleeve connection. This presents no difficulty when the external equipment uses capacitively coupled inputs. For the very small minority of equipment that is DC coupled, the LM4911 monitors the current supplied by the amplifier that drives the headphone jack’s sleeve. If this current exceeds 500mAPK, the amplifier is shutdown, protecting the LM4911 and the external equipment.
(3)
For package MUB10A, θJA = 190˚C/W. TJMAX = 150˚C for the LM4911. Depending on the ambient temperature, TA, of the system surroundings, Equation 3 can be used to find the maximum internal power dissipation supported by the IC packaging. If the result of Equation 1 or 2 is greater than that of Equation 3, then either the supply voltage must be decreased, the load impedance increased or TA reduced. For the typical application of a 3V power supply, with an 32Ω load, the maximum ambient temperature possible without violating the maximum junction temperature is approximately 144˚C provided that device operation is around the maximum power dissipation point. Thus, for typical applications, power dissipation is not an issue. Power dissipation is a function of output power and thus, if typical operation is not around the maximum power dissipation point, the ambient temperature may be increased accordingly. Refer to the Typical Performance Characteristics curves for power dissipation information for lower output powers. POWER SUPPLY BYPASSING As with any amplifier, proper supply bypassing is important for low noise performance and high power supply rejection. The capacitor location on the power supply pins should be as close to the device as possible. Typical applications employ a 3V regulator with 10mF tantalum or electrolytic capacitor and a ceramic bypass capacitor which aid in supply stability. This does not eliminate the need for bypassing the supply nodes of the LM4911. A bypass capacitor value in the range of 0.1µF to 1µF is recommended for CS.
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LM4911
POWER DISSIPATION
Application Information
LM4911
Application Information
Additionally, Mute should not be enabled during shutdown or while entering or returning from shutdown. This is not a valid operation condition and may result in much higher pop and click values.
(Continued)
MICRO POWER SHUTDOWN The voltage applied to the SHUTDOWN pin controls the LM4911’s shutdown function. Activate micro-power shutdown by applying a logic-low voltage to the SHUTDOWN pin. When active, the LM4911’s micro-power shutdown feature turns off the amplifier’s bias circuitry, reducing the supply current. The trigger point varies depending on supply voltage and is shown in the Shutdown Hysteresis Voltage graphs in the Typical Performance Characteristics section. The low 0.1µA(typ) shutdown current is achieved by applying a voltage that is as near as ground as possible to the SHUTDOWN pin. A voltage that is higher than ground may increase the shutdown current. There are a few ways to control the micro-power shutdown. These include using a single-pole, single-throw switch, a microprocessor, or a microcontroller. When using a switch, connect an external 100kΩ pull-up resistor between the SHUTDOWN pin and VDD. Connect the switch between the SHUTDOWN pin and ground. Select normal amplifier operation by opening the switch. Closing the switch connects the SHUTDOWN pin to ground, activating micro-power shutdown. The switch and resistor guarantee that the SHUTDOWN pin will not float. This prevents unwanted state changes. In a system with a microprocessor or microcontroller, use a digital output to apply the control voltage to the SHUTDOWN pin. Driving the SHUTDOWN pin with active circuitry eliminates the pull-up resistor. Shutdown enable/disable times are controlled by a combination of CB and VDD. Larger values of CB results in longer turn on/off times from Shutdown. Smaller Vdd values also increase turn on/off time for a given value of CB. Longer shutdown times also improve the LM4911’s resistance to click and pop upon entering or returning from shutdown. For a 2.4V supply and CB = 4.7µF, the LM4911 requires about 2 seconds to enter or return from shutdown. This longer shutdown time enables the LM4911 to have virtually zero pop and click transients upon entering or release from shutdown. Smaller values of CB will decrease turn-on time, but at the cost of increased pop and click and reduced PSRR. Since shutdown enable/disable times increase dramatically as supply voltage gets below 2.2V, this reduced turn-on time may be desirable if extreme low supply voltage levels are used as this would offset increases in turn-on time caused by the lower supply voltage. This technique is not recommended for OCL mode since shutdown enable/disable times are very fast (0.5s) independent of supply voltage.
PROPER SELECTION OF EXTERNAL COMPONENTS Proper selection of external components in applications using integrated power amplifiers is critical to optimize device and system performance. While the LM4911 is tolerant of external component combinations, consideration to component values must be used to maximize overall system quality. The LM4911 is unity-gain stable which gives the designer maximum system flexibility. The LM4911 should be used in low gain configurations to minimize THD+N values, and maximize the signal to noise ratio. Low gain configurations require large input signals to obtain a given output power. Input signals equal to or greater than 1Vrms are available from sources such as audio codecs. Very large values should not be used for the gain-setting resistors. Values for Ri and Rf should be less than 1MΩ. Please refer to the section, Audio Power Amplifier Design, for a more complete explanation of proper gain selection Besides gain, one of the major considerations is the closedloop bandwidth of the amplifier. To a large extent, the bandwidth is dictated by the choice of external components shown in Figures 2 and 3. The input coupling capacitor, Ci, forms a first order high pass filter which limits low frequency response. This value should be chosen based on needed frequency response and turn-on time. SELECTION OF INPUT CAPACITOR SIZE Amplifying the lowest audio frequencies requires a high value input coupling capacitor, Ci. A high value capacitor can be expensive and may compromise space efficiency in portable designs. In many cases, however, the headphones used in portable systems have little ability to reproduce signals below 60Hz. Applications using headphones with this limited frequency response reap little improvement by using a high value input capacitor. In addition to system cost and size, turn on time is affected by the size of the input coupling capacitor Ci. A larger input coupling capacitor requires more charge to reach its quiescent DC voltage. This charge comes from the output via the feedback Thus, by minimizing the capacitor size based on necessary low frequency response, turn-on time can be minimized. A small value of Ci (in the range of 0.1µF to 0.39µF), is recommended.
MUTE When in C-CUPL mode, the LM4911 also features a mute function that enables extremely fast turn-on/turn-off with a minimum of output pop and click with a low current consumption (≤ 100µA). The mute function leaves the outputs at their bias level, thus resulting in higher power consumption than shutdown mode, but also provides much faster turn on/off times. Mute mode is enabled by providing a logic high signal on the MUTE pin in the opposite manner as the shutdown function described above. Threshold voltages and activation techniques match those given for the shutdown function as well. The mute function is not necessary when the LM4911 is operating in OCL mode since the shutdown function operates quickly in OCL mode with less power consumption than mute.
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AUDIO POWER AMPLIFIER DESIGN A 25mW/32Ω Audio Amplifier Given: Power Output Load Impedance Input Level
25mWrms 32Ω 1Vrms
Input Impedance 20kΩ A designer must first determine the minimum supply rail to obtain the specified output power. By extrapolating from the Output Power vs Supply Voltage graphs in the Typical Performance Characteristics section, the supply rail can be easily found. 3V is a standard voltage in most applications, it is chosen for the supply rail. Extra supply voltage creates headroom that 16
Ci ≥ 1 / (2π * 20kΩ * 20Hz) = 0.397µF; use 0.39µF.
(Continued)
The high frequency pole is determined by the product of the desired frequency pole, fH, and the differential gain, AV. With an AV = 1 and fH = 100kHz, the resulting GBWP = 100kHz which is much smaller than the LM4911 GBWP of 10MHz. This figure displays that is a designer has a need to design an amplifier with higher differential gain, the LM4911 can still be used without running into bandwidth limitations. Figure 4 shows an optional resistor connected between the amplifier output that drives the headphone jack sleeve and ground. This resistor provides a ground path that supressed power supply hum. Thishum may occur in applications such as notebook computers in a shutdown condition and connected to an external powered speaker. The resistor’s 100Ω value is a suggested starting point. Its final value must be determined based on the tradeoff between the amount of noise suppression that may be needed and minimizing the additional current drawn by the resistor (25mA for a 100Ω resistor and a 5V supply).
allows the LM4911 to reproduce peak in excess of 25mW without producing audible distortion. At this time, the designer must make sure that the power supply choice along with the output impedance does not violate the conditions explained in the Power Dissipation section. Once the power dissipation equations have been addressed, the required gain can be determined from Equation 2.
(4) From Equation 4, the minimum AV is 0.89; use AV = 1. Since the desired input impedance is 20kΩ, and with a AV gain of 1, a ratio of 1:1 results from Equation 1 for Rf to Ri. The values are chosen with Ri = 20kΩ and Rf = 20kΩ. The final design step is to address the bandwidth requirements which must be stated as a pair of -3dB frequency points. Five times away from a -3dB point is 0.17dB down from passband response which is better than the required ± 0.25dB specified. fL = 100Hz/5 = 20Hz fH = 20kHz * 5 = 100kHz
ESD PROTECTION As stated in the Absolute Maximum Ratings, the LM4911 has a maximum ESD susceptibility rating of 2000V. For higher ESD voltages, the addition of a PCDN042 dual transil (from California Micro Devices), as shown in Figure 4, will provide additional protection.
As stated in the External Components section, Ri in conjunction with Ci creates a
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Figure 4. The PCDN042 provides additional ESD protection beyond the 2000V shown in the Absolute Maximum Ratings for the VOC output
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LM4911
Application Information
LM4911 Stereo 40mW Low Noise Headphone Amplifier with Selectable Capacitive Coupled or OCL Output
Physical Dimensions
inches (millimeters) unless otherwise noted
MSOP Order Number LM4911MM NS Package Number MUB10A
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